One characteristic of approaching estrus in animals or the "midcycle" period of the primate menstrual cycle is a rapid rise in plasma estrogens. A predictable physiologic effect of these steroids demonstrable in animal experiments is a marked increase in rate of blood flow through the uterus and vagina. This estrogen-induced hyperemia is reflected in a reduced temperature difference between these organs and the arterial blood and an elevated thermal conductance of these organs. In an attempt to understand more fully the temporal nature of these humoral, metabolic and vascular alterations in the female during the normal ovarian cycles as well as during pregnancy, we are studying the cyclic changes in thermal conductance and plasma estrogen, progesterone and LH by radioimmunoassay. Thermal conductance is determined by a variety of probes which fit comfortably in the vagina. Other thermal probes can be adopted for studying mammary skin blood flow. The work is undertaken in part to answer the question: Can ovulation be predicted or detected by the use of these thermal devices?